1. Field of the Invention
The invention relates to an extreme UV radiation source device which emits extreme UV radiation. The invention also relates to an extreme UV radiation source device in which, by rotating the discharge electrodes which constitute means for heating and excitation of extreme UV radiating fuel, the extreme UV radiating fuel is heated and excited and in which extreme UV radiation is thus produced.
2. Description of Related Art
According to the miniaturization and increased integration of an integrated semiconductor circuit, an increase in resolution is required in a projection exposure tool for purposes of its manufacture. To meet this requirement, the wavelengths of the exposure light source are being increasingly shortened. As a semiconductor exposure light source of the next generation in succession to an excimer laser device, an extreme UV radiation source device (hereinafter also called an EUV radiation source device) is being developed which emits extreme UV radiation (hereinafter also called EUV (extreme ultra violet) radiation) with wavelengths from 13 nm to 14 nm, especially with a wavelength of 13.5 nm.
Several schemes are known for producing EUV radiation in an EUV radiation source device. In one, by heating and excitation of an EUV source fuel, a high density and high temperature plasma is produced and EUV radiation is extracted from this plasma.
This EUV radiation source device is, for the most part, divided based on the method of producing a high density and high temperature plasma into an EUV radiation source device of the LPP (Laser Produced Plasma) type and into an EUV radiation source device of the DPP (Discharge Produced Plasma) type (for example, reference is made to the publication “Current status of research on EUV (extreme ultraviolet) radiation sources for lithography and future prospects” J. Plasma. Fusion Res. Vol. 79, No. 3, pp 219-260, Mar. 2003 (hereafter Publication 1).
In an EUV radiation source device of the LPP type, EUV radiation from a high density and high temperature plasma is used which is formed by irradiated targets such as solids, liquid, gas and the like which are irradiated with a pulsed laser.
On the other hand, in an EUV radiation source device of the DPP type, EUV radiation from a high density and high temperature plasma which has been produced by power current driving is used. The discharge method in an EUV radiation source device of the DPP type is a Z pinch method, a capillary discharge method, a dense plasma focus method, a hollow cathode triggered Z-pinch method and the like.
In the above described EUV radiation source devices of the two types, the radiating fuel for emission of EUV radiation with a wavelength of 13.5 nm, i.e., the fuel for the high density and high temperature plasma, is currently, for example, decavalent xenon (Xe) ions. Furthermore, the fuels for obtaining a greater radiation intensity of the EUV radiation are lithium (Li) ions and tin (Sn) ions. For tin, the ratio of the electrical input necessary to form the high density and high temperature plasma to the output of the EUV radiation with a wavelength of 13.5 nm, i.e., the EUV conversion efficiency (=EUV energy/input energy for discharge) is several times higher than for xenon. For tin and lithium, the EUV conversion efficiency is also greater than in xenon. Therefore, tin and lithium are regarded as best for use as the radiating fuel of an EUV radiation source with a large output.
The EUV radiation source of the DPP type compared to the EUV radiation source of the LPP type has the advantages of enabling a small EUV radiation source device and a small power consumption. Practical use in the market is strongly expected.
On the other hand, as was described above, in an EUV radiation source device of the DPP type, EUV radiation from a high density and high temperature plasma which has been produced by power current driving as a result of a discharge is used. The means for heating and excitation of the EUV radiating fuel is a large current as a result of a discharge which has formed between a pair of discharge electrodes. The discharge electrodes are therefore exposed to a high thermal load as a result of the discharge. Since high density and high temperature plasma forms in the vicinity of the discharge electrodes, the discharge electrodes are also exposed to a thermal load of this plasma.
The discharge electrodes are gradually worn off by this thermal load, by which metallic debris forms. In the case of using an EUV radiation source device as the radiation source device of an exposure tool, EUV radiation which is emitted by the high density and high temperature plasma is focused by means of EUV collector optics and this focused EUV radiation is emitted to the side of the exposure tool. The metallic debris damages the EUV collector optics and adversely affects the EUV radiation reflection factor of the EUV collector optics.
The gradual wearing off of the discharge electrodes furthermore changes the shape of the discharge electrodes. Because of this, the discharge which forms between the discharge electrodes gradually becomes unstable. As a result, also the generation of the EUV radiation becomes unstable. This means that the lifetime of the discharge electrodes, in the case of using an EUV radiation source device of the DPP type as the radiation source of a semiconductor exposure tool of the mass production type, must be extended as much as possible.
To meet this need, in the publication “EUV sources using Xe and Sn discharge plasmas ”J. Phys.D. Appl. Phys. 37 (2004) 3254-3265 (hereafter, Publication 2), an EUV radiation source device of the DPP type with discharge electrodes with a long lifetime which reacts accordingly to the above described circumstances is suggested. FIG. 5 shows a radiation source device as is described in Publication 2.
The EUV radiation source device shown in FIG. 5 has a chamber 1 as the discharge vessel in which there are a discharge part 2 as means for heating and excitation which heats and excites the EUV radiating fuel, and an EUV collector part 3 with EUV collector optics 3a which focuses the EUV radiation which is emitted by the high density and high temperature plasma which has been produced by heating and excitation of the EUV radiating fuel by the discharge part 2.
The EUV collector part 3 focuses the EUV radiation and guides it from an EUV radiation extracting part 4 located in the chamber 1 into the irradiation optical systems of an exposure tool which is not shown in the drawings. The chamber 1 is connected to a pumping device 5. The interior of the chamber 1 is subjected to a decompression atmosphere by this pumping device.
(a) Discharge Part
The discharge part 2 has an arrangement in which a first discharge electrode 2a of a metallic disk-like component and a second discharge electrode 2b likewise of a metallic disk-like component are arranged such that an insulating material 2c is clamped by them. The middle of the first discharge electrode 2a and the middle of the second discharge electrode 2b are arranged essentially coaxially. The first discharge electrode 2a and the second discharge electrode 2b are attached in a position which is apart by the thickness of the insulating material 2c. In this connection the diameter of the second discharge electrode 2b is greater than the diameter of the first discharge electrode 2a. Furthermore the thickness of the insulating material 2c, i.e. the distance between the first discharge electrode 2a and the second discharge electrode 2b, is roughly 1 mm to 10 mm.
The second discharge electrode 2b is provided with the rotating shaft 6a of a motor 6. In this connection, the rotating shaft 6a is mounted essentially in the middle of the second discharge electrode 2b such that the middle of the first discharge electrode 2a and the middle of the second discharge electrode 2b are located essentially coaxially on the axis of rotation. The rotating shaft 6a is for example routed via a mechanical seal 6b into the chamber 1 which allows rotation of the rotating shaft 6a while maintaining a decompression atmosphere within the chamber.
On the bottom of the second discharge electrode 2b, a first sliding contact 7a and a second sliding contact 7b, for example, of carbon brushes or the like, are mounted. The second sliding contact 7b is electrically connected to the second discharge electrode 2b. On the other hand, the first sliding contact 7a is electrically connected to the first discharge electrode 2a via a through opening 2e which penetrates the second discharge electrode 2b. An insulating arrangement (not shown) prevents formation of an insulation breakdown between the first sliding contact 7a which is electrically connected to the first discharge electrode 2a, and the second discharge electrode 2b. 
The first sliding contact 7a and the second sliding contact 7b are electrical contacts which, during sliding, still maintain electrical connections and they are connected to a pulsed power generator 7 which supplies pulsed power via the first sliding contact 7a and the second sliding contact 7b between the first discharge electrode 2a and the second discharge electrode 2b. 
This means that, when the first discharge electrode 2a and the second discharge electrode 2b are rotated by the motor 6, pulsed power from the pulsed power generator 7 is applied between the first discharge electrode 2a and the second discharge electrode 2b via the first sliding contact 7a and the second sliding contact 7b. 
The pulsed power generator 7 applies a pulsed power with a short pulse width via a magnetic pulse compression (MPC) circuit part of a capacitor C and a magnetic switch SR between the first discharge electrode 2a and the second discharge electrode 2b which constitute a load. Line routing from the pulsed power generator 7 to the first sliding contact 7a and to the second sliding contact 7b takes place also via an insulating current feed terminal (not shown), which is installed in the chamber 1 and which, while maintaining the decompression atmosphere within the chamber 1, enables electrical connections of the pulsed power generator to the first sliding contact 7a and the second sliding contact 7b. 
The peripheral areas of the first discharge electrode 2a and the second discharge electrode 2b of metallic disk-like components are made edge-shaped. As is described below, between the edge-shaped regions of the two electrodes, a discharge forms when power is applied by the pulse power generator 7 to the first discharge electrode 2a and the second discharge electrode 2b. When the discharge forms, the vicinity of the electrodes reaches a high temperature. The first discharge electrode 2a and the second discharge electrode 2b therefore are made of a metal with a high melting point, such as, for example, tungsten, molybdenum, tantalum or the like. The insulating material 2c is, for example, silicon nitride, aluminum nitride, diamond, or the like. In the peripheral area of the second discharge electrode, there is a groove area 2d which also reaches a high temperature when the discharge forms.
Solid Sn or solid Li, which is the fuel for the high temperature plasma, is supplied to the discharge part 2. The fuel can be supplied as solid Sn or solid Li that has been attached beforehand to the groove area 2d or also by the fuel supply unit 8.
The motor 6 turns only in one direction. By operating the motor 6, the rotating shaft 6a turns, and the second discharge electrode 2b and the first discharge electrode 2a which are mounted in the rotating shaft 6 turn in one direction.
The Sn or Li, which has been attached in the groove area 2d of the second discharge electrode 2b or which has been supplied to it is moved by rotation of the second discharge electrode 2b to the exit side of the SUV radiation in the discharge part 2.
On the other hand, in the chamber 1, there is a laser irradiation apparatus 9 which irradiates the Sn or Li which has been moved toward the side of the above described EUV collector part with a laser. The laser is a YAG laser or carbon dioxide gas laser or the like.
The laser of the laser irradiation apparatus 9 is emitted via a chamber window (not shown), which is located in the chamber 1, and a laser focusing means onto the Sn or Li which has moved toward the side of the above described EUV collector part, as focusing radiation.
As was described above, the diameter of the second discharge electrode 2b is greater than the diameter of the first discharge electrode 2a. The laser can therefore be easily aligned such that it passes through the side of the first discharge electrode 2a and irradiates the groove area 2d of the second discharge electrode 2b. 
The emission of the EUV radiation from the discharge part 2 takes place as follows:
The laser is emitted from the laser irradiation apparatus 9 onto the Sn or Li of the groove area 2. The Sn or Li irradiated with the laser vaporizes between the first discharge electrode 2a and the second discharge electrode 2b, part thereof being ionized.
If, in this state, between the first and second discharge electrodes 2a, 2b, a pulsed power with a voltage between roughly +20 kV to −20 kV is applied by the pulse power generator 7, a discharge arises between the edge-shaped areas which are provided in the peripheral areas of the first discharge electrode 2a and the second discharge electrode 2b. In this connection, a pulse-like heavy current flows in the partially ionized part of the Sn or Li which is vaporized between the first discharge electrode 2a and the second discharge electrode 2b. Afterwards, a high density and high temperature plasma is formed by vaporized Sn or vaporized Li by Joulean heating by the pinch effect in the peripheral area between the two electrodes. This high density and high temperature plasma emits EUV radiation.
Since, between the first and the second discharge electrodes 2a, 2b, a pulsed power is applied as was described above, the discharge becomes a pulsed discharge. The emitted EUV radiation becomes pulsed radiation which is emitted in a pulse-like manner.
(b) EUV Radiation Collector Part
The EUV radiation emitted by the discharge part 2 is focused by means of the EUV collector optics 3a of the oblique incidence type which is located in the EUV radiation collector part 3 and is delivered to the irradiation optical system of an exposure tool which is not shown in the drawings by the EUV radiation extracting part 4 located in the chamber 1.
The EUV collector optics 3a has several mirrors which are formed, for example, in the shape of an ellipsoid of rotation with different diameters or in the form of a paraboloid of rotation with different diameters. These mirrors are arranged coaxially such that the center axes of the rotation come to rest on one another so that the focal positions essentially agree. These mirrors can advantageously reflect EUV radiation with an oblique angle of incidence from 0° to 25° by, for example, the reflection surface of the substrate material having a smooth surface of nickel (Ni) or the like, or being densely coated with a metallic layer of ruthenium (Ru), molybdenum (Mo) or rhodium (Rh).
(c) Debris Trap
Furthermore, there is a debris trap 10 between the above described discharge part 2 and the EUV radiation collector part 3 to prevent damage to the EUV collector optics 3a. The debris trap 10 mitigates and catches debris, such as metallic powder/particles or the like, which are produced by the high density and high temperature plasma by sputtering of the peripheral areas of the first and second discharge electrodes 2a, 2b which are in contact with the high density and high temperature plasma, debris as a result of Sn or Li as radiating fuel and the like and passes only EUV radiation.
In the EUV radiation source device of the DPP type shown in FIG. 5, the debris trap 10 is comprised of a gas curtain 12 and a foil trap 11. The gas curtain 12 is formed by gas which is supplied to the chamber 1 via a nozzle from a gas supply unit 14.
FIGS. 6(a) and 6(b) each schematically show the gas curtain device. As is shown in FIG. 6(a), a nozzle 14a has the shape of a rectangular parallelepiped and its opening from which gas is emitted has a narrow rectangular shape. When gas is supplied from the gas supply unit 14 of the nozzle 14a, gas is emitted sheet-like from the opening of the nozzle 14a, by which a gas curtain is formed. The gas curtain 12 changes the feed direction of the above described debris and restrains its reaching the EUV collector optics 3a (shown downstream of gas supply unit 14 in FIG. 1). The gas used in this connection for the gas curtain is preferably gas with a high transmittance to EUV radiation such as, for example, one of the rare gases, such as helium, argon or the like, and also hydrogen or the like.
FIG. 6(b) shows a diffuser 14b for drawing in the above described gas; it is located opposite the above described nozzle 14b and intakes the gas emitted from the nozzle.
Furthermore, between the gas curtain 12 and the EUV collector optics 3a, there is the foil trap 11 which is called such in published patent application JP-OS 2004-214656 (U.S. Patent Application Publication 2004-0184014 A 1) (hereafter, Patent Document 1). The foil trap 11 is comprised of several plates which are arranged in the radial direction of the area in which the high density and high temperature plasma has formed, such that the EUV radiation emitted by the high density and high temperature plasma is not screened, and of a ring-like support body which supports these plates, and is mounted on a partition 13 of the foil trap. By placing such a foil trap 11 between the gas curtain 12 and the EUV collector optics 3a, the pressure between the high density and high temperature plasma and the foil trap 11 increases, raising the gas density of the gas curtain 12 present at this location and thus collisions between the gas atoms and the debris increase. By repeated collisions, the debris reduces its kinetic energy, by which the energy decreases in the collisions of the debris with the EUV collector optics 3a. This makes it possible to reduce damage to the EUV collector optics 3a.
By the above described EUV radiation source device of the DPP type, the position changes for the two electrodes 2a, 2b on which a pulse discharge arises for each pulse, since the first discharge electrode 2a and the second discharge electrode 2b rotate. The thermal load to which the first discharge electrode 2a and the second discharge electrode 2b are subject is therefore reduced, by which the rate of wear of the discharge electrodes is reduced. Thus, a lengthening of the lifetime of the discharge electrodes 2a, 2b is enabled.
FIG. 5 shows the discharge part 2 as if it were larger than the EUV radiation collector part 3, but it is shown as such only for simplification of understanding. The actual size ratios therefore do not correspond to FIG. 5.
As was described above, in the formation of EUV radiation, a pulsed power with a voltage of roughly +20 kV to −20 kV is applied between the first and second discharge electrodes 2a, 2b. On the other hand, the distance between the first and second discharge electrodes 2a and 2b is roughly 1 mm to 10 mm, for example, 5 mm. The peripheral areas of the first and second discharge electrodes 2a, 2b are formed in the shape of edges, and the electrical field is concentrated in the edge-shaped areas.
Therefore, the pressure in the discharge part 2 in a decompression atmosphere must be kept, for example, at a few Pa or less than or equal to this value. Otherwise, in the second discharge electrode 2a also outside the vicinity of the area which is irradiated with a laser by the laser irradiation apparatus 9 a discharge forms between the edge-shaped areas which are provided in the peripheral areas of the first and second discharge electrodes 2a, 2b. 
The state in which a discharge forms outside the vicinity of the laser-irradiated area leads to an increase of the thermal load on the electrodes. The rate of wear of the discharge electrodes 2a, 2b is not reduced. Furthermore, the discharge becomes unstable in the vicinity of the laser-irradiated area, by which the EUV output also becomes unstable.
On the other hand, on the debris trap 10 located between the discharge part 2 and the EUV radiation collector part 3, for example, of the gas curtain 12 and foil trap 1, the kinetic energy of the debris cannot diminish if the pressure is not somewhat high. The effect exerted on the EUV collector optics 3a by the contamination cannot be suppressed. If an attempt is made to suppress the effect of the contaminants, the region in the radiation exit direction proceeding from the debris trap 10 (i.e., the area in which the debris trap 10 and EUV collector optics 3a are located) has altogether, for example, roughly a few hundred Pa.
This means that the area proceeding in the radiation exit direction in front of the debris trap 10 has roughly a few 100 Pa because in the EUV radiation source device of the DPP type shown in FIG. 5 for suppressing the effect of the contaminants a gas curtain 12 and a foil trap 11 are used as the debris trap 10. Then the pressure within the chamber 1 in the discharge part 2 exceeds a few Pa. When the pressure within the chamber 1 exceeds a few Pa, a discharge forms in unwanted regions of the first and second discharge electrodes 2a, 2b, as was described above. Therefore, it is necessary to carry out differential pumping such that the pressure in the discharge part 2 is subjected to a decompression atmosphere, for example, of a few Pa, and moreover, the area in which the debris trap 10 and the EUV collector optics 3a are located is subjected to a pressure atmosphere of a few 100 Pa.
For example, in published International Patent Application WO 2005/025280 A2 (hereafter, Patent Document 2), between the rotating discharge electrodes and the EUV collector optics, there are screens with openings, and thus, it is indicated that the pressure on the sides of the discharge electrodes is made into a lower pressure by the openings becoming pressure resistances.
FIG. 7 shows the arrangement of the EUV radiation source device described in Patent Document 2. FIG. 7 is a cross section of the EUV radiation source device shown in FIG. 1 of this publication. Disk-like electrodes 34, 36 rotate around an axis of rotation 46. Heated, liquid metal 47 is used as the fuel for the high density and high temperature plasma. The electrodes 34, 36 are partially immersed in it. The liquid metal 47 which is in contact with the surfaces of the electrodes 34, 36 is transported by rotation of the electrodes 34, 36 to a discharge part 45 with a given gap and is caused to vaporize by laser 48.
By application of a discharge voltage from the pulsed power generator 49 via the liquid metal 47 to the electrodes 34, 36, in the discharge part 45 a discharge is started, by which a high density and high temperature plasma is formed. The EUV radiation formed by the high density and high temperature plasma is extracted via the foil trap 50 toward the top of the drawing.
Furthermore, the screens 44 with openings on the exit side of the EUV radiation are arranged such that a space is surrounded by them contains the above described disk-shaped electrodes 34, 36 and a unit for supply of fuel 47 for the plasma. In Patent Document 2, it is indicated that the pressure on the sides of the discharge electrodes is made into a decompression by the openings of these screens becoming pressure resistances.
Assuming that, based on the suggestion described in Patent Document 2, in the device shown in FIG. 5, an effort is made to place screens with openings between the discharge part 2 and the debris trap 10, it is in fact difficult to fix the pressure in the discharge part 2 to a few Pa and to subject the region in which the debris trap 10 and the EUV collector optics 3a are located to a pressure atmosphere of roughly a few 100 Pa.
As was described above, in the discharge part 2, the discharge produces a high density and high temperature plasma which has such a high temperature that the first discharge electrode 2a and the second discharge electrode 2b of a material with a high melting point are worn off. It is therefore necessary to move the screens with the openings a certain amount (by a few cm) away from this high density and high temperature plasma.
On the other hand, the openings of the screens have regions through which the EUV radiation, which is emitted by the high density and high temperature plasma, passes. The size of the respective opening must be fixed at a value at which at least the EUV radiation which is incident in the EUV collector optics is not screened.
For example, if the openings of the screens which are moved a few cm away from the high density and high temperature plasma are set to a size at which the EUV radiation incident in the EUV collector optics is not screened, and if the distance between the discharge electrodes (i.e., the size of the resulting high density and high temperature plasma) is fixed, for example, at 5 mm, the respective opening reaches a size of roughly 10 cm. It is imagined that, for the openings with this size, the pressure in the discharge part 2 can be set to roughly a few Pa and for the region in which the debris trap 10 and the EUV collector optics 3a are located can be maintained at a pressure atmosphere of a few 100 Pa.
Under the above described pressure conditions, the gas located within the chamber is in a viscous flow state. Via the opening, the gas located on the debris trap 10 flows to the side of the discharge part 2. If the above described pressure conditions should prevail, it is therefore necessary to arrange a large pumping device with a large pumped amount at the side of the discharge part 2.
In the discharge part 2, laser yields vaporized Sn gas or vaporized Li gas. It is required of the pumping device to be placed in the discharge part 2 that pump treatment of this metal vapor can be carried out. This means that, on the side of the discharge part 2, a pumping device is required which has a large pumped amount and in which, moreover, pump treatment of the metal vapor is possible. This pumping device is large, costly and therefore not realistic.
On the other hand, metallic debris which forms when generating a high density and high temperature plasma by the discharge, and debris as a result of the EUV radiating fuel pass through the openings of the screens. Therefore, it is necessary, as shown in FIG. 6(a) & 6(b), to fix the gas curtain such that at least the overall openings are covered.
In the gas curtain device shown in FIGS. 6(a) & 6(b), the gas flows sheet-like in the vicinity of the openings. However, the flow becomes more of a turbulent flow the farther it is from the openings. Therefore, gas curtain loses its velocity in the center of the openings and flows onto the side of the discharge part when the gas supplied from the gas supply unit 14 does not have a somewhat high flow velocity. For this reason, in the openings, a region is formed in which there is hardly any gas curtain. The debris which has passed through this area damages the EUV collector optics 3a since the kinetic energy is not much reduced by the foil trap 11. As a result, the gas supply amount per unit of time from the gas supply unit 14 can be increased.
However, when the openings of the screens are roughly 10 cm, as was described above, it is in fact very difficult, even using a large pumping device, to maintain the pressure of one of the screens at a few Pa and the pressure of the other screen at a few 100 Pa, i.e., at this pressure difference. Therefore, a pressure difference between the two sides of the screens cannot be formed. If a pressure difference cannot be formed, a gas curtain cannot be used as a debris trap since, by using a gas curtain, the pressure in the discharge part increases, and thus, a discharge forms in unwanted regions of the first and second discharge electrodes.